A low noise amplifier is typically used to amplify the received signal from an antenna at microwave frequencies. A block diagram of a system which uses a single stage low noise amplifier (LNA) as a first receiver block in such a system is shown in FIG. 1(a). An antenna sub-assembly 1 is comprised of an antenna 3 which feeds a low noise amplifier 5. A coaxial cable carries an amplified signal from the LNA to a receiver unit 9, and in particular typically to an anti-aliasing RF filter 11, which feeds its output to an RF amplifier 13 of the receiver unit.
FIG. 1(b) is a block schematic of a system which uses a two stage LNA in the antenna sub-assembly. An antenna 3 feeds a first stage LNA 5, which feeds an anti-aliasing filter 11, which feeds an RF amplifier 15. The output of amplifier 15 is carried by a coaxial cable 7 to a receiver unit 17, which applies its input signal to an amplifier mixer, etc.
GAAS and PHEMPT GAAS FET transistors are currently widely used in LNAs at frequencies of 1 GHz and higher. Such devices are of relatively low cost and offer very low noise and high gain at moderate currents and voltages.
The LNA is generally wideband relative to the signal bandwidth and usually does not impose limitation on signal modulation or architecture on the balance of the receiver system. For example, the LNA could be used for a narrow band quadrature phase-shift key (QPSK) system, or for a wideband direct sequence spread spectrum system, provided only that any in-line filters have sufficient bandwidth to pass the entire signal spectrum (as is the usual case).
Power consumption of the individual stages of such amplifiers is typically 10 mA from a 5V power supply; multiple stages increase the current draw proportionally. While this current draw is considered to be moderate as compared with earlier technology, it represents a substantial drain for battery powered equipment such as hand held global positioning system (GPS) receivers. It would therefore be desirable to reduce the current consumption.
A PHEMPT FET, when operated at a drain current of about 10 mA, has a negative gate to source voltage typically between 0.1V and 0.4V. If the source is grounded, it becomes necessary to bias the gate negatively with respect to ground to achieve the desired bias current. This is commonly achieved by the used of capacitive pump circuits which generate negative bias voltages. The PHEMPT gate input impedance (at DC) is very high and thus the input bias current is very low and the bias circuit current consumption can be made relatively low.
Variation in the source to gate threshold for GAAS FET transistors is not well controlled and consequently, additional control circuitry is required to regulate the bias current which flows in the circuit. Commonly, the negative bias voltage provided by the capacitive pump circuit simply provides the necessary biasing voltages and additional circuitry is required to implement the bias current control.
FIG. 2(a) is a schematic diagram which shows a means of biasing a PHEMPT FET without a negative bias pump. An FET receives an RF input signal at its gate. A high value resistor 23 is connected between the gate and ground, and another resistor 25, bypassed by a capacitor 27, is connected between its source and ground. A power source is connected to ground and is coupled to the drain of the FET.
This circuit relies on a degeneration resistor 25 connected to the source to control the bias current. A major disadvantage of this simple circuit is that the variation in gate threshold for PHEMPT FET devices is very poor, leading to wide tolerance of current draw.
FIG. 2(b) illustrates a biasing circuit which makes use of a negative bias device. An FET 21 has its input AC coupled (e.g. via capacitor 29) to the RF input. Its source is grounded. A capacitive pump 31 generates a DC voltage negative with respect to ground and provides it from its output to the gate of the FET via resistor 33. Capacitor 35 AC bypasses the output of pump 31 to ground.
However, in this case where a two stage LNA is to be employed, at least double the single stage typically 10 mA current is drawn.
While GAAS FETs and PHEMPT GAAS FETs are capable of operation at extremely high frequencies, it is important to provide well controlled AC source impedances at all ports up to the maximum frequency of operation to prevent spurious oscillations. For that reason, to achieve such control it is common practice to connect the GAAS FET source directly to the ground plane.